The present invention relates to the art of intrusion detection. More particularly, it relates to improvements in intruder detection systems of the so-called "dual technology" variety.
Heretofore, a variety of "technologies" have been used to detect the presence of an intruder in region under surveillance. Microwave, ultrasonic, photoelectric and passive infrared are some of the more common technologies in current use. Each has certain unique advantages and disadvantages which makes it more or less desirable for a particular environment or application. None is fool-proof, and all are subject to the ever-annoying false alarm.
In the never-ending struggle to provide the perfect intruder detection system, "perfect" from the standpoint that it never false alarms, proposals have been made to combine two (or more) technologies in a common intruder detection system. See, for example, the disclosure of U.S. Pat. Nos. 3,725,888; 3,801,978; 4,243,979; 4,275,390; 4,331,952; 4,660,024 and 4,401,976. While such proposals go back at least twenty five years (see, e.g., U.S. Pat. No. 3,074,053), only recently has the cost of electronics reached a level that has made commercialization of a "dual-tech" system viable.
In conventional dual-technology systems, the outputs of the different intruder-detecting subsystems (e.g. microwave and passive infrared subsystems) are fed to an AND gate or its equivalent. Only in the event that the outputs of both subsystems indicate that both subsystems have detected intrusion substantially simultaneously, or within a predetermined time interval, will the AND gate provide an alarm-activating signal. The advantage of such a system, of course, is that false alarms will only occur on the relatively rare occasion that a spurious or false alarming-producing event is detected by both subsystems at about the same time. By combining relatively diverse technologies, e.g. microwave and photoelectric or passive infrared, the probability of false alarming can be minimized.
While conventional dual technology intruder detection systems have, indeed, proven to be substantially more reliable and less susceptible to false alarming than "single technology" systems, they may still, given the right circumstances, false alarm. Consider, for example, the conventional passive IR/microwave dual technology system. Here, the microwave component transmits microwave radiation throughout a field of view which is both broad (e.g., 60 degree angular range) and deep (e.g., up to 500 feet in range), and looks for changes in frequency of the transmitted radiation, as produced by the well-known Doppler effect. Meanwhile, the passive IR subsystem monitors a plurality of discrete, narrow fields of view for temperature changes, as produced by the body heat of an intruder. Being passive in nature and looking for extremely small changes in temperature, the detection range of the IR component is substantially shorter than that of the active (microwave) subsystem, perhaps only about 10% as long. In installing such dual technology systems, there is a great tendency for the installer to ignore certain sources of false alarms of the IR subsystem even when there is reason to suspect that they are within one or more of the fields of view of such subsystem. The rationale for ignoring such sources is that, since the microwave component will not detect such spurious IR sources, there is no need to cure the problem, such as by either masking the optics of the troublesome field(s) of view of the IR subsystem or, alternatively, installing an entirely different type of system having different, presumably non-troublesome, fields of view. The fallacy of this rationale, of course, is that even though the microwave subsystem will not respond to temperature changes, it has its own peculiar set of false-alarm-producing sources, any one of which can produce a system false alarm if it occurs at substantially the same time as the spurious IR source occurs.
It is known in the intruder detection art to provide some passive IR detection systems with modular optical systems which facilitate quick changes in the optical pattern of protection. In such systems, the particular pattern of protection depends on the orientation of one optical component relative to another. For example, the Model DS 964 Passive Infrared Detection manufactured by assignee, Detection System, Inc., includes a movable, multifaceted reflector module comprising two set of individual planar reflectors. The position of this module is variable with respect to a stationary spherical reflector element to provide two different patterns of protection. Typically, one set of facets are arranged to provide so-called "barrier" protection, each of the individual fields of view being arranged in a common, say, vertical plane, and the other set of facets are arranged to provide broader coverage, in which case the facets are directed in different directions and in different planes. As of the date of the invention hereof, such modular optical systems have not been employed in the afore-described dual technology systems. As noted above, there did not seem to be a need for such.